Recently, digital cameras which photograph an object using a solid-state image sensing device such as a CCD (Charge Coupled Device) array in lieu of a silver-salt film are becoming popular. As for digital cameras in general, still cameras exist for still photography and video cameras exist for motion picture photography.
Recently, miniaturization of CCD elements has been progressing rapidly, and this miniaturization has caused a demand for photographic lenses that have a shorter overall length and a simple construction. A conventional photographic lens for a digital camera is described, for example, in Japanese Laid Open Patent Application No. 2000-180719. This publication discloses photographic lenses of five-piece and of four-piece construction. However, conventional photographic lenses, including the lens described in this publication, are still insufficient in terms of simplicity of construction and shortness of overall length.
In addition, one cannot say that photographic lenses developed thus far for digital cameras have excellent performance, especially in terms of the wideness of the angle of view. This is because, when the photosensitive element is a CCD array rather than a silver-salt film, there is a restriction in the angle of view due to a loss of detector efficiency that occurs with CCD detectors when the incident rays are non-normal to the image surface of the detector. For example, the lens described in the above-mentioned patent publication, when scaled for a camera using standard 35 mm film, would have a focal length of 40 mm. On the other hand, a wide-angle lens for standard 35 mm film typically has a focal length of about 28 mm. Thus, there remains a high demand for development of a photographic lens for digital cameras which has a wide angle of view, in addition to simplicity of construction and a short overall length.
The present invention relates to a photographic lens for use in a digital camera. More particularly, the object of the invention is to provide a wide-angle, single focus lens that forms a bright image while having a simple construction and short overall length.